The present invention relates to the control of exhaust gas recirculation (EGR) in internal combustion engines for motor vehicle use. In automotive engine applications, fluid pressure actuated controllers have been employed for moving a valve member, or pintle, for controlling flow of exhaust gas through a passage interconnecting the combustion chamber of exhaust and intake passages in the engine. Such fluid pressure valve controllers are often provided with a diaphragm actuator having a rod connected from the diaphragm to the pintle to move the pintle in response to a fluid pressure signal. A signal is provided to the diaphragm actuator indicative of varying engine load conditions. A convenient source of such a signal is ported or raw engine manifold vacuum, exhaust backpressure or a combination of manifold vacuum and exhaust gas backpressure. It is also known to combine engine inlet manifold vacuum and throttle venturi suction signals to provide a control signal for operating the exhaust gas recirculation valve or pintle.
Known examples of EGR controllers employing a combination of engine manifold vacuum and exhaust backpressure are the devices described in U.S. Pat. Nos. 4,116,182, 3,799,131 and 3,762,384. The '182 patent utilizes a device employing a combination of exhaust gas backpressure and manifold vacuum to provide a variable percentage of EGR; whereas, the '131 and '384 patents employ a combination of manifold vacuum and exhaust backpressure to provide a generally constant percentage of EGR. Another EGR controller employing ported manifold vacuum in combination with exhaust backpressure to control EGR is shown and described in U.S. Pat. No. 3,880,129.
Another known EGR valve controller is that known in the art as the sonic controller, which employs complex electromechanical sensors and actuators for moving an EGR valve pintle so as to maintain sonic flow at the valve seat, whereby the flow is independent of pressure variations downstream of the pintle. Such a sonic type EGR valve controller provides desired control of EGR; however, the electromechanical sensors and control systems required to sense the pintle position and move the pintle for maintaining sonic flow across the valve seat have been quite costly and difficult to manufacture for repeatability and reliability. Consequently, EGR controllers operating to maintain sonic flow across the EGR pintle have not found widespread acceptance.
One of the problems encountered in providing the proper amount of EGR for the required reduction in exhaust emissions has been that the EGR valve controllers responding to engine manifold vacuum to provide control of the flow of EGR, have under certain operating conditions caused engine faltering and poor drivability. This problem has been particularly troublesome where the EGR controller, responding to increased engine manifold vacuum caused by throttle closure at high RPM, provided a large dose of EGR. Upon reopening of the throttle faltering and loss of power occurred in the engine.
It is also known to utilize engine manifold vacuum as a fluid pressure power source for the EGR valve actuator and to superimpose thereon the effects of carburetor venturi suction pressure change, which varies inversely as manifold vacuum, to provide control of EGR valve movement. Carburetor venturi suction alone is a low level signal with insufficient power to drive an EGR valve controller. However, combining carburetor venturi suction and manifold vacuum in a single controller results in a complicated mechanism and requires the costly additional hoses and pressure tap at the carburetor venturi. Furthermore, many carburetors in automotive engine applications have variable venturi throat configurations and thus a pressure tap at the venturi throat would not read a pressure directly proportional to engine mass flow.
Engine manifold vacuum is nevertheless a convenient source of fluid pressure which varies in accordance with changes in engine load and speed and is thus desirable as an input signal for an EGR controller. Furthermore, engine manifold vacuum is readily accessible and provides the lowest cost source of an input control signal for an EGR flow control valve. Thus, it has been desirable to provide a means of controlling EGR in an engine utilizing manifold vacuum as the primary control signal and yet to provide a way of preventing overdosage of EGR at engine operating conditions in which the manifold vacuum reaches high levels, but the engine load is such that minimum or no EGR flow is required for reduction of exhaust emissions.